1. Technical Field
This invention relates in general to hydrodynamic bearings and in particular to an apparatus and method for testing a hydrodynamic thrust bearing in a disk drive spindle motor.
2. Background Art
In hydrodynamic lubrication theory, it is well known that the coefficient of friction is velocity dependent. This relationship is known as the Stribeck curve (FIG. 1). For example, when the rotational speed of a fluid hydrodynamic bearing (FDB) is low, the friction torque is high due to solid contact between the elements of the bearing. This is the "boundary lubrication" area indicated in FIG. 1. However, as the rotational speed is increased, a fluid film begins to form which decreases friction. Since the fluid film is not fully developed at this stage, the contact between the components is intermittent. This second stage is known as "mixed lubrication." In the final stage, the bearing has a critical or "take off" speed where the fluid film becomes fully developed and friction is at a minimum. This last stage is known as "full fluid lubrication." Thereafter, friction gradually increases as a linear function of viscous drag.
The take off speed 10 of one type of journal bearing in a disk drive spindle motor is illustrated in FIG. 2. Take off speed is a function of bearing design, surface roughness, sufficient oil, excessive tilt, and external load. If the take off speed for a particular bearing is higher than its design level, a defect of some sort is present in the assembly. Thus, bearings may be tested in this manner in the assembly line to assure their quality prior to assembly into the end product.
In the prior art, an apparatus for testing hydrodynamic rotary or journal bearings has been developed. In this apparatus (FIG. 3), an object such as a disk drive spindle motor 11 containing journal bearings 13 and a thrust bearing 15, has a coaxial drive shaft 17 that is rigidly mounted to a stationary torsion transducer 21 for rotation therewith. A load motor 23 rotates the motor 11 relative to the shaft 17 by applying a transverse or side load to spindle motor 11 through a flexible belt 25. Load motor 23 is mounted to a linear stage 27 which generates selected radial load forces from load motor 23 to spindle motor 11. A load cell 29 monitors the load exerted by linear stage 27. Unfortunately, this method is only capable of testing the take off speed of journal bearings 13, not that of the thrust bearing 15. Thus, a method and apparatus for testing the take off speed of a thrust bearing is needed.